They're made differently than most of us imagine, without havin' studied 'em a bit more closely ... we have diaphrams, whereas they rely upon the movement of their bodies to breathe. And, they have air sacs that ... rather than explaining? There's a really good online interactive presentation of the
Anatomy of the Chicken >
> peck here << to open it's introductory page in a new window ~'-)
That's gonna help you become familiar w/ what's on the inside, which is clearly where she's hurt. As for worries of pain? They're built very differently in the way that works as well ... they don't suffer nearly so much as we do, just by havin' to watch what hurts us to see.
And, not to be tellin' how to handle your own, but kids 'n pets tend to be accidents in the makin' when it comes to chickens -- they rarely mean to cause 'em any harm.
Another way to figure out what's goin' on is by comparing this one most carefully w/ the way others feel (most esp. if you've got birds of the same breed/sex/age, as folks so often do ~'-)
Some dislocations and fractures can be manipulated, and I'll try 'n walk you through whatever you can find, if I can. Be sure 'n update this thread when you have or need new information.
You should become familiar w/ tube feeding in advance, just in case it comes to that.
Also, you can, and most probably should, give your bird aspirin: Dissolving one baby aspirin into a quart of water would be about the right amount, as the normal solution calls for 5 five-grain tablets per gallon, which is 324 mg/gal. It's to be offered free choice, and result in a daily dosage of about 25 mg/lb of body weight.